There are cases where different sectors transmit the same signal, namely a sector common signal, in a communication system such as a cellular system. The different sectors may be located in the same base station or in different base stations, respectively.
One example of the sector common signal is a broadcast channel. All sectors in the cellular system may use the common signal for the broadcast channels to cancel interference and to obtain a frequency diversity gain. However, transmit antenna beam patterns may be distorted when all sectors in one base station transmit the common signal. When the different sectors in the same base station transmit the common signal, the transmit beam pattern for the common signal generally corresponds to the sum of the values resulting from multiplying an antenna pattern of each sector by a weight. The weight can be given as a time-invariant complex value with random phase. In the worst case where the phase difference between the weights for two sectors in the same base station is 180 degrees, the power of a signal to be transmitted toward the direction of sector boundary where two antenna beams overlap decreases continuously. Accordingly, the performance of the cellular system may be significantly deteriorated.
Another example of the sector common signal is a synchronization channel. When a terminal initially detects a sector having the strongest received power or periodically searches neighboring sectors during operation, the synchronization channels are used. Since a primary synchronization channel among the synchronization channels is used for a terminal to synchronize with a base station, the number of sequences available for the primary synchronization channel is limited lest the configuration of the terminal become complicated. Accordingly, the occurrence of the case where neighboring sectors use a common sequence as the primary synchronization channel is inevitable. In the case where the different sectors use the common sequence as the primary synchronization channel, when the terminal coherently detects a sequence of a secondary synchronization channel by using the channel estimated by the primary synchronization channel, the performance of a search unit is deteriorated. On the other hand, the neighboring sectors may use a common sequence as the secondary synchronization channel according to the arrangement of sectors. For example, the cellular system may be designed such that the different sectors in the same base station use the common sequence as the secondary synchronization channel and use the different primary synchronization channels. In the case where the common sequence is used for the synchronization channel, the transmit antenna beam patterns may be distorted as in the broadcast channel.